Ever wonder why some people can sleep soundly through a rumbling thunderstorm? These self-proclaimed heavy sleepers are difficult to wake because even their alarms fail to disrupt their slumber. Meanwhile, the light sleepers are riled up by the smallest sounds — like birds tapping on the window or leaves rustling outside – even if they lay on the most comfortable mattress.

Not even scientists have a definite explanation. But they have a few theories as to why some sleep heavily and some sleep lightly.

REM and Non-REM Sleep

One of these theories is that light and heavy sleepers experience different duration of REM (Rapid Eye Movement) and Non-REM sleep.

All sleeping cycles are divided into two: Non-REM and REM. Non-REM covers the period from wakefulness to deep sleep. Then comes REM, which occurs about 90 minutes after you’ve fallen asleep. Breathing becomes faster and irregular, and the heart rate and blood pressure increase. Most of your dreams occur during REM.

Heavy sleepers might spend more time in the deep sleep stage of Non-REM compared to light sleepers. This means that they’re less vulnerable to environmental stimuli and less likely to wake up. This could also explain why old people are more likely to be light sleepers. Young people generally spend more time in deep sleep. As they grow older, they spend less time in the deep-sleep stages, which makes them light sleepers.

The Number of Sleep Spindles

Researchers also cite the activity in the brain to explain the heaviness of sleep. Sleep researchers from the Harvard Medical School and Massachusetts General Hospital investigated brain-wave patterns related to how noises wake people up.

Lead researcher Dr. Jeffrey Ellenbogen studied the sleeping patterns of 12 individuals for three nights. The research team bombarded the participants with 14 different noises during the study, ranging from car traffic to slamming doors to airplane engines. These were played at louder volumes as the study progressed.

The researchers found that the thalamus, the part of the brain that processes visual and auditory stimuli, produces varied numbers of pulses (sleep spindles) among the participants. Those who were able to sleep through sounds (heavy sleepers) recorded more sleep spindles than those who were easily woken up.

They concluded that people who produced more sleep spindles are more likely to insulate themselves from environmental stimuli and, consequently, experience deeper sleep. This information could help people with fewer sleep spindles find ways to protect themselves from disturbances during their sleep, like wearing earplugs or sound-proofing their rooms.

Moreover, Dr. Ellenbogen hopes that further research can lead to the development of drugs that enable better sleep. Future treatments might be able to manipulate the number of sleep spindles, which could help light sleepers block out stimuli.

One thing doesn’t change, however. Whether you’re a light or heavy sleeper, you need a healthy amount of sleep in order to function well. Some lifestyle changes don’t depend on you being a light or heavy sleeper. By following a consistent bedtime routine, exercising, and appropriate meal times, you can enjoy deeper, more restful sleep.